1. Field of the Invention
The present invention relates to an industrial fabric having uniform drainage characteristics throughout the fabric, which is obtained by reducing a water retention amount in the inner space of the fabric during papermaking without changing the thickness of a wire or fabric, thereby retaining required characteristics of the fabric such as drainage property, surface properties, and rigidity and at the same time, suppressing transfer of marks onto a paper made using the fabric, generation of splash, or the like.
2. Description of the Related Art
Fabrics obtained by weaving warps and wefts have conventionally been used widely as an industrial fabric. They are, for example, used in various fields including papermaking fabrics, conveyor belts, and filter cloths and are required to have fabric characteristics suited for the intended use or using environment. Of such fabrics, a papermaking fabric used in a papermaking step for removing water from raw materials by making use of the mesh openings of the fabric should satisfy a severe demand.
For example, there is a demand for the development of papermaking fabrics that have excellent surface smoothness so as not to cause transfer of a wire mark of the fabric to paper, have a drainage property and filterability (air permeability) to sufficiently and uniformly dehydrate excessive water contained in the raw materials, have enough rigidity and wear resistance that enable suited use even under severe environments, and can maintain conditions necessary for making good paper for a prolonged period of time.
In addition, industrial fabrics are required to have a fiber supporting property, improved papermaking yield, dimensional stability, running stability, and the like. In recent years, owing to the speed-up of a paper making machine, requirements for papermaking fabrics have become severer.
Most of the required characteristics of industrial fabrics and solutions thereof can be understood from a description on papermaking fabrics on which the most severe demand is imposed among industrial fabrics. Therefore, a description will next be made with papermaking fabrics as an example.
With a recent increase in the speed of a papermaking machine, papermaking fabrics are required to have a particularly excellent drainage property and surface smoothness. Although drainage characteristics which they are required to have are different with the type of a papermaking machine or the type of a product to be manufactured, a uniform drainage property is one of necessary conditions which any product should have.
Further, it becomes more difficult to satisfy the required characteristics of papermaking fabrics because an increase in the mixing rate of minute fibers in raw materials as a result of recent increased use of waste paper causes insufficient drainage so that sufficient and uniform drainage has gained importance (see, for example, Japanese Patent Application Publication No. 2004-68168).
In order to satisfy such required characteristics, the fabric having an improved drainage property has conventionally been obtained mainly by providing a drainage trench on the lower surface side of a papermaking fabric. For example, improvement of the drainage property has been achieved by decreasing the diameter of wefts to increase their density, thereby reducing loss of fibers and overlapping upper side warps with lower side warps in a perpendicular direction.
Upon papermaking, however, water to be removed is inevitably retained in the inner space of the papermaking fabric and interferes with drainage. In addition to this problem, splash at the folded-back end portion of the papermaking fabric in a papermaking machine contaminates the periphery of the machine.
For example, FIGS. 13 and 14 are design diagrams showing one example of a currently used industrial fabric of a related art. The term “design diagram” as used herein means a minimum repeating unit (which may be called as a “complete design”) of a fabric pattern and this complete design is horizontally and perpendicularly connected to each other to form the entire fabric pattern.
FIG. 14A is a schematic view of the related art showing the longitudinal section taken along an upper side warp (1u) and a lower side warp (1d) of FIG. 13. FIG. 14B is a schematic view showing the longitudinal section taken along an upper side warp (2u) and a warp binding yarn (2b) of FIG. 13. FIG. 14C is a schematic view showing a cross section taken along an upper side weft (1′u) and a lower side weft (1′d) of FIG. 13.
In the design diagram, warps are indicated by Arabic numerals, for example, 1, 2, 3 . . . and 8. The warps are composed of an upper side warp and a lower side warp or in some places, an upper side warp and a warp binding yarn. As shown in FIGS. 14A-14C, upper side warps are indicated by numerals with u, lower side warps are indicated by numerals with d, and warp binding yarns are indicated by numerals with b. For example, as shown in FIG. 14A, the warp 1 is composed of an upper side warp (1u) and a lower side warp (1d).
Wefts are indicated by Arabic numerals with prime, for example, 1′, 2′, 3′ . . . 8′. Depending on the arrangement ratio of wefts, there are sites where an upper side weft and a lower side weft are placed perpendicularly and sites where only an upper side weft is placed. As shown in FIG. 14, upper side wefts are indicated by numerals with u and lower side wefts are indicated by numerals with d. For example, weft 1′ is composed of an upper side weft 1′u and a lower side weft 1′d. 
In the design diagrams of FIGS. 1, 3, 5, 7, 9, 11 and 13, the symbol “x” shows that an upper side warp (u) is located on an upper side weft (u) and forms a knuckle on the upper side surface of the upper side fabric; the symbol “▴” shows that a warp binding yarn (b) interweaves with an upper side weft (u) and forms a knuckle on the upper side fabric; the symbol “∘” shows that a lower side warp (d) is located below a lower side weft (d) and forms a knuckle on the lower side surface of the lower side fabric.
In the industrial fabric shown in FIG. 13, upper side wefts (u) and lower side wefts (d) overlap in a perpendicular direction at (upper side weft (u)):(lower side weft (d)) arrangement ratio of 2:1. This means that the upper side wefts (u) from 1′ to 8′ are arranged, while lower side wefts (d) of 1′, 3′, 5′, and 7′ are arranged. According to the design diagram, yarns are arranged perpendicularly while being overlapped exactly for convenience of the diagram, but they may be misaligned in actual fabrics.
As shown in FIG. 14A, an upper side warp (1u) goes over one upper side weft (1′u), goes under three upper side wefts (2′u to 4′u), goes over one upper side weft (5′u), and goes under three upper side wefts (6 to 8′u) and thus they are interwoven. On the lower surface side, on the other hand, a lower side warp (1d) goes under one lower side weft (1′d) and goes over three lower side wefts (3′d, 5′d, 7′d) and thus they are interwoven.
In such a constitution, as shown in FIG. 14C, a portion lacking in, between the upper side weft (1′u) and the lower side weft (1′d), the upper side wefts (2u, 3u, 4u, 6u, 7u, 8u), the lower side warps (3d, 4d, 5d, 7d), and the warp binding yarns (2b, 6b) becomes an inner space (S). Upon papermaking, water to be removed is retained in such an inner space (S).
Improvement in a drainage property has conventionally been achieved mainly by providing a drainage trench on the lower surface side of a papermaking fabric. Even if a trench is provided on the lower surface side, it is impossible to overcome the problem because when water to be removed is retained in the inner space of the industrial two-layer, it disturbs drainage and moreover, causes splash.
In order to reduce the inner space, a simple increase in the amount of yarns in the inner space can be considered, but an increase in the number of yarns in the inner space may cause a new problem, that is, an increase in the thickness of a wire or fabric compared with that of the conventional fabric.